A Fresnel lens is a lightweight and compact flat lens constructed by replacing the curved surface of a convex lens or a concave lens with a series of discontinuous curved surfaces formed by a plurality of prisms arranged concentrically or in parallel, thereby reducing the lens thickness to the minimum required to achieve the necessary curved surface.
Fresnel lenses are widely used to convert a light beam from a point light source into a parallel beam of light, such as a lens used with a backlight in a liquid crystal display, or conversely to concentrate a parallel beam of light into a defined beam, such as a condensing lens used in a solar power generating system.
In a Fresnel lens having a small F-number (focal length/lens diameter), the angle of light deflection (known as the angle of deviation) is larger at the lens peripheral regions located farther away from the optical axis. In such regions, since the angle of the slanted face of each prism is large, and the angle of incidence on the refracting interface is therefore large, reflections at the refracting interface increase and the transmittance decreases. To address this, it has been known to improve the transmittance in a segment where a large deviation angle is required by designing the vertex angle of the prism so that the incident light is first totally reflected at the slanted face of the prism and then refracted at the perpendicular face, and such Fresnel lenses are disclosed, for example, in U.S. Pat. No. 4,755,921, Japanese Unexamined Patent Publication No. 2002-221605 and U.S. Pat. No. 4,337,759.